Hot gas turbines have been widely used since their inception and first introduction approximately fifty years ago. In particular, they are found in many stationary applications where a continuous and reliable source of power is required, such as to generate of electrical power at remote locations, to pump gas or petroleum at high volume for extended periods of times, including many months, or even years. They also find many applications in transportation such as jet engines, marine propulsion, and tractor trailer combinations or big rigs. They vary in size from several thousand kilowatt hours per day, as in oil field operations, to generate electrical pumping power, or for gas injection and the like tip to many megawatts as in electric power generation and distribution. Although hot gas turbines are highly efficient overall, and relatively economical to operate, they range in cost from $500,000 to a few millions dollars, for modestly sized units, such as those that generate a few hundred kilowatts per day. Larger stationary hot gas turbine units, generating several hundred thousand kilowatt hours per day may cost from several million dollars up to several tens of millions of dollars.
It has long been known that the efficiency of such hot gas turbines is highly dependent upon full utilization of power generated by combustion of hydrocarbon liquids or gas with high pressured air generated by the air compressor section of a gas turbine engine. In its general configuration, a hot gas turbine comprises an air compressor section having a multiplicity of air compressor stages, that compress air to several hundred pounds per square inch, as it successively passes through the several compressor stages. The compressor stages are driven as a unit by a single common drive shaft, that extends through a combustion zone where hot gas is generated by the compressed air and fuel to drive the turbine section at the other end of the common drive shaft. The hot gas section is made tip of a plurality of hot gas stages each of which includes a turbine blade disc directly mounted on the common drive shaft. Power from the drive shaft is delivered to either one, or both ends, through suitable gearing, to drive any rotatable load, such as an electrical generator, an auxiliary pump, or the like.
While it has been known for a number of years that the efficiency of a hot gas turbine can be improved if clearance between the tips of the rotating blades on each turbine disc and the surrounding tip shoes can be reduced to a minimum so as to prevent hot, high pressured gases from bypassing each set of turbine blades by flow around the outer edges or tips of the blades. Unfortunately, for greatest efficiency, the rotor blades must be operated at high temperatures and high speeds that cause thermal and centrifugal forces that radially expand such turbine blades. Additionally, gravity, work loads and vibration forces acting on the turbine discs tend to cause the blade tips to "carve" or erode away portions of the surrounding tip shoes. In particular, the lower portion of a ring of the surrounding tip shoes, as viewed from the center line of the rotating common drive shaft may permit the blade tips to cut a non-circular path to form such an oval track over an arc of from a few degrees to a hundred degrees or more of the circumference.
Because of these changes in path of the blade tips in going from cold, or ambient, conditions to normally high operating temperatures of 900.degree. to 1850.degree. F. and at high speeds, clearance, or "gaps," between the blade tips and the tip shoes can vary widely around the circumferential path. To avoid potential damage to the disc blades, or possible destruction of the turbine hot stage, and even ballistic destruction of the casing or anything external around its circular path, we have found that factory or overhaul gaping is frequently too wide to achieve ideal or "rated" clearance so that no possible contact will occur between the two surfaces. And although abradable materials have been known, both for coating the ends of the turbine blades and the tip shoes, such materials have generally been used only on the air compressor stages which are not subjected to temperature changes of several hundred degrees, as compared to the hot stage turbine blades and tip shoes. Furthermore, where such abradable material has been added to either the blade tips or the tip shoes, the clearance is normally set so that little or no abrasion occurs. This appears to be due to a fear that such abraded material from the first or second hot stage discs may damage or plug the nozzles of a subsequent turbine stage.
Accordingly, little or no attempt seems to have been made to adjust the clearances between the turbine blades and tip shoe after the turbine is installed and operating. Thus, the only alternatives to operation at less than rated power, has been to keep running at the same low-efficiency, or to rent a replacement turbine, or to shut-down and ship the entire engine to an overhaul, or factory repair facility. Furthermore, correction of the gaping has not always assured that such an overhaul will in fact increase the power output of the installed turbine. The costs of shutting down or renting a substitute engine, and the cost of shipping and repair of the turbine can frequently exceed 100,000 dollars to several hundred thousand dollars. Where the gas turbine is located in a remote location, it is of course even more costly and more time consuming to have the engine packaged, and sent to be overhauled. Hence, there has long been a need for a field repair or overhaul procedure for improving the hot gas section of a gas turbine. However, there are other reasons which appear to have prevented people from attempting such field overhaul. These relate to a fear of possible injury of people or equipment, if any of the hot gas stages fail mechanically. A particular danger in this regard is that in factory assembly or overhaul, it has been considered essential to dynamically rebalance the common drive shaft, including both the air compressor assembly and the hot gas assembly. Since each of these assemblies is made up of a multiplicity of discs and their multiplicity of blades, such initial balancing, or subsequent rebalancing present a complex vibration problem. That problem relates to the axial spacing of and tile two assemblies from each other and their wide range of speeds, as well as the flexibility of the common drive shaft.
The length of tile drive shaft and tile separation of such rotating masses on the drive shaft requires careful dynamic balancing of the rotating components as a unit, at up to their maximum operating speeds. Such balancing is to avoid serious damage of the hot gas turbine blades due to vibration at any speeds over which the common drive shaft may operate. Since the same drive shaft also is coupled to a rotatable load either at a single end or at both ends of the drive shaft, any rotational vibrations of the drive shaft present an exceedingly difficult problem to handle.
For the foregoing reasons it will be understood that although there have been great incentives to develop a field reliable method, or procedure, for overhauling the hot gas section of a gas turbine engine, such need has not been answered prior to our invention. In carrying out our invention, we have devised methods, apparatus, and materials, that permit such economic rehabilitation of a stationary, or a transport, gas turbine engine without danger of modifying the dynamic balance of a turbine drive shaft during such repair or overhaul including all of its various components at any location, world wide. At the same time such overhaul or rehabilitation of the hot gas section is performed without removal of the complete gas turbine engine, from its normal outer package or its interconnections with its rotatable load. Accordingly, repair and installation of modified, and more effective, tip shoes, including abradable materials substantially reduce clearance between the tip shoes and the turbine blade tips is now possible and thereby substantially improves the power output and overall efficiency of the gas turbine engine. At the same time, prolonged down time of the turbine installation, is significantly reduced to a period of time to not more than a few hours, to a day or two. Thus overhaul, for complete disassembly, and assembly, including replacement of parts, and re-assembly for return to service, are all performed with minimum danger of damage to the turbine or personnel in the vicinity of the turbine.